An array antenna used in radar systems includes a plurality of individually radiating antenna elements. In some array antennas, the individual antenna elements are coupled to a transmitter through phase shifters and attenuators configured for controlling the phase and amplitude of the transmitted signal. Similarly, the individual antenna elements are coupled to a receiver through phase shifters and attenuators configured for controlling the phase and amplitude of the received signal. A device comprising both a transmitter and a receiver which are combined and share common circuitry is here referred to as a transceiver. The relative phase and amplitude of the radio frequency signal passing between the plurality of antenna elements and a corresponding plurality of individual transceiver elements are controlled to obtain a desired radiation pattern. The pattern obtained is a result of the combined action of all the individual transceiver and antenna elements.
In the past, radars were used to transmit and receive radio waves having only a single polarization. As a consequence, a target which can reflect only a singly polarized beam perpendicular to the incident polarized beam has the potential of being invisible, even if a target has a strong reflection coefficient.
Polarimetric systems (also referred to as “dual polarization systems”) have been used primarily because of their properties regarding signal to clutter enhancement or improved target classification and identification. Polarimetric radars transmit and receive both horizontal and vertical polarizations. Beams having horizontal polarization provide essential information about horizontal “properties” of the target, whereas vertically polarized beams provide essential information about vertical “properties” of the target. Since the power returned from the radar is a complicated function of the target size, shape, orientation, density, reflectivity, etc, the additional information received from the second type of polarization can provide improved target detection.
A monopulse radar technique and/or a radar interferometric technique can be used to gather angle information about a target, for example, when used in a tracking radar.
The basic monopulse radar system uses four antennas, or four quadrants of a single antenna that are controlled together. The target is illuminated by all four quadrants, and a comparator network is used to produce four return signals. These return signals include a “sum” signal (Σ) that is a combination of the received signal from all four quadrants, an elevation angle difference signal (ΔE) that is formed by subtracting the signal from the two upper quadrants from the signal from the two lower quadrants and an azimuth angle difference signal (ΔA) formed by subtracting the signals from the left quadrants from the signals from right quadrants. In a tracking radar, the sum signal is used to track the target's distance from the monopulse radar system and the azimuth difference signal is used to determine the target's position to the left or right of the radar system. The elevation difference signal may be used to determine the target's position relative to the horizon.
A radar interferometer is a receiving system that determines the angle of arrival of a wave by a phase comparison of the signals received at separate antennas or separate points on the same antenna.
Monopulse phased array systems are known in the art. These systems include a number of antenna elements arranged in an array. Each of the antenna elements is connected to a T/R (transmitter/receiver) module through a corresponding transmitting/receiving channel, which is under the control of a beam steering system. The beam steering system is fed by a transmitting signal from the T/R module for forming a transmitting beam. Upon reception of reflected signals, a sum signal, an elevation difference signal, and an azimuth difference signal are taken from the beam steering system. The phased array system includes a combination unit that combines the signals received from all the antenna elements and derives a total sum signal (Σ), a total elevation angle difference signal (ΔE) and a total azimuth angle difference signal (ΔA) from which regulation signals (ΔE/Σ) and (ΔA/Σ) for re-steering the transmitting beam generated under the control of the beam steering system can be obtained.